If this is Hibiscus syriacus, then it needs minimal pruning in late winter or early spring. Just cut out damaged or crossed over branches.

beat me to it spritz. though a neighbour cut his down by 2/3rds. it didnt flower that summer but it has recovered well now.

In addition to the excellent advice from spritz, I remove any small growths low down plus any branches growing into the middle of the shrub to keep it from getting congested. If I need to reduce it, I take off a complete branch rather than cutting it back

They're such slow growers! My two are nowhere near needing a prune!

Thaks to everyone who helped with my question.